In recent years, manual operations of apparatuses within vehicles, as well as in other devices, are increasingly being replaced by motor-driven operations that utilize electric motors. Motor-driven operations provide many benefits, such as space-savings, ease of assembly, improved controllability and the like. The replacement of an automatic transmission range switching mechanism within a vehicle is an example of such a replacement from manual operation to motor-driven operation. Such a mechanism is equipped with an encoder that is synchronized with the motor and outputs a pulse signal at every given angle of rotation of the motor. In operation, during a range switching (i.e., when a gear shift position of the automatic transmission is changed), the motor is driven to a target rotation position that corresponds to a target shift range and the automatic transmission is shifted to the target shift range.
In such a range switching device, for example, a shift range stop system may be provided to prevent the inadvertent switching of the shift range. The shift range may be limited by the engagement of a rotatable detent plate with a detent spring which is driven by the motor, as disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. JP-A-2004-308848). In such a system, at the time of shift range switching, the power supply for the motor is stopped when the detent plate is rotated to a target shift range. That is, the power supply to the motor is stopped when the detent plate is rotated within a safe range where the shift range is unlikely to be wrongly/inadvertently moved to a different range position during the time when the power supply to the motor is stopped.
Patent document 1 provides a shift range switching device in which (i) a motor is rotated to a target rotation position according to a feedback control (F/B) control that drives the motor by sequentially switching power supply phases of the motor based on an encoder count value, and (ii) a power supply for the motor is stopped by ending the F/B control when the motor rotates within a predetermined stop range.
However, in such a system, even when the F/B control ends and the power supply to the motor stops, the motor may still rotate to a target rotation position if the motor has already been rotated within an attractive force range of the detent mechanism or if inertia causes the motor to rotate within such range. Rotation within an attractive force range may cause rotation to the target rotation due to a latch within the detent mechanism that is biased to slide downwardly into and engage a bottom of a range detention concave part (i.e., a concave detent). That is, even after stopping the power supply to the motor, the motor may rotate to the target rotation position due to the downward sliding of the latch into a bottom of the range detention concave part.
However, when the attractive force range of the detent mechanism is narrow relative to a stop range of the motor, depending on the shape of the detent mechanism, the motor may be unable to rotate within the attractive force range of the detent mechanism at the time the F/B control ends, which also stops the power supply to the motor. That is, in such a case, the motor may be unable to rotate to the target rotation position. A solution to such a situation may be to narrow the stop range of the motor according to the attractive force range of the detent mechanism. However, such a narrowing of the stop range of the motor may cause an overshoot of the motor. That is, if the stop range of the motor is narrowed, the rotation position of the motor may be close to the target rotation position when ending the F/B control (i.e., when the power supply for the motor is stopped), thereby making the motor susceptible to over-rotation past the target rotation position.